The invention relates to a torch for use in plasma arc welding or cutting operations, employing either a non-transferred or a transferred arc.
The prior art embraces designs wherein a cylindrical electrode is attached rigidly to the plasma torch and connected to the negative pole of the electrical power supply by a conductor. The anode takes the form of a cap, likewise rigidly attached to the torch, which covers the projecting end of the electrode. The cap is insulated from the electrode, and connects with the positive pole of the power supply via a second conductor, a fine gap being created between cap and electrode through which plasma-forming gas is directed under pressure toward an orifice in the cap itself; the pierced cap thus constitutes a constricting nozzle.
In a first conventional type of torch, an arc is struck between the positive nozzle and the negative electrode utilizing a particularly high auxiliary voltage, which in most instances will be generated by complex and costly high frequency circuitry to which the torch is connected by long, heavily insulated cable.
What occurs, in practice, is that one generates a spark between the electrode and the nozzle, which occupy fixed positions in relation to one another; an arc is struck, and continues to discharge until such time as the control button or power supply switch is operated. Accordingly, the arc is struck between components of the torch, and not transferred to the work.
The expedient most widely adopted to permit transferred arc operation of such a torch is that of connecting the work (welding or cutting) to the same positive terminal as that to which the nozzle return lead is connected, and wiring-in a resistance between the return lead and terminal. Thus, when the torch is moved toward the weld or cut, the arc encounters less resistance from the work than from the return lead, and jumps across to the work.
The same effect can be produced by wiring-in switch contacts, rather than a resistance, which will be broken when the weld or cut is commenced.
In a second conventional type of torch, the arc is struck between the electrode and the work; here, the electrode is rigidly attached to and shielded by the torch and connected to the negative terminal of the power supply, whereas it is the work to be welded or cut, and not the nozzle, that is connected to the positive terminal.
The cap, or nozzle, is no longer rigidly attached in this type of torch, and an arc is struck by bringing the nozzle into momentary contact with the electrode and the work, sandwiching it swiftly between the two and then releasing. This physical contact between electrode, nozzle and work short-circuits power through the torch, whereupon, with the electrode distanced from the nozzle, the gap created between electrode and nozzle produces an arc, and the arc is transferred from the electrode to the work.
It will be appreciated that good initial contact between nozzle and work is a fundamental requirement in plasma welding or cutting with a transferred arc; this in turn dictates that the work must offer a sufficiently clean surface, signifying that a certain amount of preparation is called for, particularly where there may be rust, old paintwork, etc. . .
Transferred arc equipment must therefore incorporate special safety circuits to prevent high voltage from reaching the electrode unless the electrode itself is faultlessly positioned for operation, i.e. in contact with the work; without such a precaution, the high welding voltage would invest the operator, who will normally be in physical contact with the work.
Such a safety feature is set forth in EP No. 159256, which discloses the interposition of a low voltage circuit between the electrode and the work to be welded or cut, or rather, between the two relative terminals, the output signal from which operates a control medium designed to switch the main high voltage circuit into safe condition.
With this type of protection, the operator can proceed to transfer the arc in absolute safety, and the conventional on/off button can be eliminated from the torch; all that remains is for the operator to move the torch toward the work and establish the requisite contact between electrode, nozzle and workpiece.
The object of the invention disclosed is that of embodying a plasma welding/cutting torch which is safe to use, and which will permit of operating in non-transferred arc conditions without the need for costly ignition systems with high frequency circuits that are typical of conventional embodiments.